find, locate, whereis, which, type

I suspect that most Linux admins know 3 or 4 of these five commands, and regularly use 2 or 3 of them.

linuxhaxor has a useful introduction to all five, with the most common uses for each of them.

Note that locate requires a regular run of updatedb – the article says that “The database is automatically created and updated daily” which is true for most distributions, but it depends on your cron setup – you can update the locate db as frequently as you wish. Another thing to note about locate is that it will not use the (normally root-generated) database to tell you (as a non-privileged user) about files which you would not otherwise know about.